I found an interesting issue in SharePoint 2013 when you try to create a new Document Set based on a Content Type that has a Date and Time field on it in a site where the regional settings are configured to English United Kingdom and the time format is 24 hours. Specifically, you will receive an error that the date is in an incorrect format when you press Save. A quick trip to the forums and I received confirmation that it is a bug and that it was fixed in the March 2013 PU which can be obtained from the links below: